The present invention relates to vacuum switch assemblies which are used as an electrical shunting switch with electrochemical cells. An electrochemical or electrolytic cell is one in which a direct current is passed through an electrolyte containing solution between spaced electrodes, with ionic separation of the positive and negative ions taking place at the respective electrodes. The most common electrochemical cells are used to produce sodium hydroxide and chlorine from brine. The cells may be mercury cells in which mercury serves as the cathode, or the more modern diaphragm or membrane cells. A diaphragm or membrane cell has a porous member through which the electrolyte passes between the electrodes. The typical electrochemical cell manufacturing installation has many cells electrically in series, with a direct current working voltage for each cell of less than about 5 volts, and with a very high current of about 30,000 amperes or greater.
When a single series cell must be inspected or worked on for maintenance, it is necessary to shunt the cell to permit the continued operation of the other series connected cells. The electrical shunting switch must be capable of carrying and interrupting the very high currents of the system. The shunting switch must interrupt current in the shunt path when the cell is to be placed back in series with the other cells. Because of the high current a significant amount of energy must be dissipated during interruption. This causes switch contact deterioration and limits the switch life. The vacuum switch assembly of the present invention is compact and is portable or movable for connection to any one of the cells which make up an operating line.
A recent innovation has been the use of vacuum switches as a shunting switch for electrolytic cells, as described in copending applications, Ser. No. 650,322, filed Jan. 19, 1976, entitled "Low Voltage Switch"; Ser. No. 650,406, filed Jan. 19, 1976, entitled "Low Voltage Switch And Operating Mechanism"; and in U.S. Pat. No. 3,950,628, issued Apr. 13, 1976. The copending applications describe a vacuum switch assembly with a plurality of electrically paralleled switches which are approximately simultaneously operated as a shunting switch for an electrolytic cell. Such vacuum switch assemblies offer many practical operating advantages over the heretofore used air switches. The vacuum switch has a significant longer operating lifetime due to the greater energy dissipation characteristic.
Recently proposed electrolytic cells have operating currents which are significantly higher, some as high as 150 kiloamperes. While the vacuum switches and the operating mechanism described in the aforementioned copending applications provide approximately simultaneous opening of the parallel vacuum switches, it is really not possible with an electromechanical system to have the contacts part at the exact same instant in the context of of milliseconds, which is the time scale for arc interruption. The last switch contacts that part or open will be carrying the total current in the shunt and could be subject to gross contact erosion. It would be highly desirable to reduce the energy which must be dissipated by the last switch to open.
It has been the practice to use a single large bus conductor between the cell electrodes and the shunting switch contacts because of the high current it must carry.
Variation in the vacuum switch size and geometry offers some capability for operating at higher current ratings, but this is limited because the arc which forms during interruption occurs over a small local area of the contact.
The vacuum switch operating mechanism described in the copending applications did not permit adjustment of the operating mechanism travel to facilitate easy adjustment of switch openings to ensure that they were approximately simultaneous.